1. Field of Invention
This invention relates generally to the mechanical and electrical structure of fluid drop ejectors.
2. Description of Related Art
A conventional thermal inkjet transducer array is essentially a large bank of thin-film resistive heaters electrically connected in parallel. In particular, a thermal inkjet printer comprises an array of drop ejectors. Each drop ejector has an ink channel having an inlet end and a nozzle end and contains a resistive heater. The nozzle end of each resistive heater in the array of drop ejectors is connected to a common electrical bus, which in turn is connected to an electrical power supply providing a printer operating voltage. Each individual drop ejector is driven to eject a droplet of ink by grounding an inlet end of the resistive heater through an individually-addressable driver transistor.
Often, fluid ejection systems, such as inkjet printers, include an array of thin-film drop ejectors electrically connected in parallel. A drop ejector includes a channel into which fluid flows, a resistive heater to vaporize a portion of the fluid to form a bubble in the chamber, and a nozzle through which fluid downstream of the vapor bubble ejects from the chamber to form a drop projected towards a receiving medium. Vaporizing the fluid creates pressure in the channel forcing the fluid collected downstream of the heater out of the nozzle.
Each drop ejector in the array connects to a common electrical bus communicating with an electrical power supply. Each ejector is controlled by grounding an electrical supply end through an individually-addressable driver transistor. Design optimization encourages a narrow electrical bus for minimal nozzle length, and thick cross-section structures to minimize electrical resistance. Such a thick bus, underneath the channel (˜6-7 μm high), can present an obstacle over which the fluid must flow. Such a flow restriction can induce lateral forces to the fluid being ejected, creating potential directional biases and/or variations in the drop and/or satellite trajectories, thereby degrading ejection quality.
The common electrical bus should be narrow, so that the length of the ink nozzle can be kept as short as possible. This tends to increase drop ejection energy efficiency. To reduce the electrical series resistance of the common bus, it is desirable to make the common bus relatively thick. Often, the common bus will have two or more layers of metal and/or polysilicon. However, this thick bus structure presents a “bump”-shaped obstacle in the nozzle that misdirects the ejected main drop and/or associated satellite droplets that are ejected with the main drop. The misdirected satellite drops tend to limit the print quality achievable with drop ejectors having this bump-shaped obstacle. Unfortunately, no reasonable alternative to these drop ejectors was previously available.
A fluid ejector having a low topography formed by rerouting the electrical conductors from underneath to adjacent to the chamber is disclosed in U.S. Pat. No. 6,227,657 (the 657 patent), which is incorporated herein by reference in its entirety. This low topography fluid drop ejector provides for an electrical contact structure to the resistive heater that avoids placing relatively thick electrical contact layers in a fluid drop ejection path between the resistive heater and the ejector nozzle.